Kinetics and mechanism of oxidation of vanillin by Cerium(IV) in aqueous perchlorate medium

Abstract

The oxidation reaction of vanillin with cerium (IV) in perchloric acid medium has been studied spectrophotometrically over the range 5.0 ≤ 10³[Vanillin]_T ≤ 25.0; 0.3 ≤ [H⁺]_T ≤ 0.8; <i style="mso-bidi-font-style:normal">I</i> = 1.0 mol dm^-3 (NaClO₄) and 293 ≤ T ≤ 308K. The rate of the reaction has been found to increase with the increase in [H⁺]_T and [Vanillin]_T. The reaction follows first order kinetics with respect to each [Ce(IV)]_T, [Vanillin]_T and fractional order with respect to [H⁺]_T. The reaction proceeds in two paths and the values of <i style="mso-bidi-font-style:normal">k₁</i> and <i style="mso-bidi-font-style:normal">k₂ </i>have been calculated using a suitable rate law. The ΔH^# (kJ mol^-1) for <i style="mso-bidi-font-style:normal">k₁</i> and <i style="mso-bidi-font-style:normal">k₂</i> paths are 3.45 ± 0.24 & 43.61 ± 2.49 and ΔS^# (JK^-1 mol^-1) for <i style="mso-bidi-font-style:normal">k₁</i> and <i style="mso-bidi-font-style: normal">k₂</i> paths are –247.81 ± 0.81 & –141.23 ± 8.26 respectively. High negative value of activation entropy is indicative of ordered transition state for the reaction. The product of the reaction has been found to be 4-hydroxy-3-methoxybenzoic acid.